Power control systems for electronically commutated motors (ECM), sometimes referred to as brushless direct current (DC) motors, may advantageously utilize pulse width modulation (PWM) techniques for controlling motor operation. In general, such systems employ controllable power switching devices such as, for example, power transistors, silicon controlled rectifiers (SCR) or gate turn-off devices (GTO), serially connected between a power source and appropriate terminals of the motor. For a three phase motor, the system may utilize a three phase bridge arrangement with each of the three motor power terminals being connected to a corresponding leg of the three phase bridge. Each leg of the bridge may include a series connected pair of switching devices, one of the devices being operable to connect the motor terminal to a positive voltage source for supplying current to the motor and the other of the devices being operable to connect the motor terminal to a negative voltage source for allowing current to circulate out of the motor. Each switching device is responsive to a gating signal for becoming conductive and allowing current to pass in the associated winding phase of the motor. The gating signals are coupled to selected ones of the switching devices in an ECM control system in a manner to energize the windings of the motor in a predetermined sequence. In a PWM system, either a current monitoring circuit or a voltage control circuit is effective to generate drive enable signals when motor current and/or voltage is less than a predetermined value. The driven enable signal, which is the pulse width modulated signal and is hereinafter referred to as the PWM enable signal, allows the gating signals to be coupled to the appropriate switching devices. Removal of the PWM enable signal inhibits coupling of the gating signals to the power switching devices.
Pulse width modulation of the drive enable signal may be used to establish an average voltage or a desired motor current. Since motor torque is a function of motor current, torque can be controlled by regulating current so long as adequate voltage is available. Since motor speed is a function of average motor voltage, speed can be controlled by regulating average voltage so long as adequate current rating is available in the power devices. In systems using PWM for voltage control, the voltage coupled to a load is defined by the PWM duty cycle multipled by the available supply voltage. The switching frequency, i.e., the cycle time, is generally constant so that voltage regulation requires only a determination of the ratio of conduction to non-conduction time during each cycle.
PWM systems which regulate current have required continuous monitoring of load current to avoid uncontrolled high frequency switching, or have exhibited discontinuity in control output when load current approaches the regulated value, or have not provided for a smooth transition from a current control mode to a voltage control mode. If load current cannot be continuously monitored, such as, for example in a full wave bridge switching circuit for an ECM motor with back EMF rotor position sensing, a means for controlling the interval of the off or non-conduction time is required to prevent excessive high frequency switching. Two common methods employ either a free-running oscillator or a monostable timer. The free-running oscillator is used to establish a fixed maximum frequency of operation. The monostable timer is used to establish a fixed off time. The circuit which establishes a fixed maximum frequency of operation utilizes a flip-flop clocked by the free-running oscillator. The oscillator generates clock signals which establish the PWM cycle. This first type circuit exhibits a discontinuity when load current is near a regulated value since the PWM enable signal will occur in near coincidence with the oscillator clock signals and result in alternate cycles being off. Another type circuit uses a monostable timer to latch a flip-flop and effect a drive enable delay. This second type circuit creates a fixed off time and introduces a problem in transitioning from current to voltage control since voltage control generally requires a fixed integration interval for developing a time integral of voltage applied to a load such as an ECM.
Each of these circuits have their advantages and disadvantages. The fixed frequency oscillator is the best means for average voltage control; however, unless other means are provided, this control approach produces a discontinuity in control output as a current regulator when the current approaches the regulate value at near 100% on time of the fixed period. Even if other means are used to avoid the discontinuity at near 100% on time, the off interval is not fixed but is the remaining interval of the fixed period of the oscillator, which will often result in inadequate time for the decay of the inductive stored current in the load to permit a sustained turn on at the beginning of the next oscillator period. The fixed off time one shot is the best means for current control; however, it does not provide a smooth transistion to voltage control and can at light load conditions lead to higher than desired switching frequency.
The free-running oscillator approach can be improved by inserting a fixed off-time interval at the end of each oscillator cycle. This off-time will avoid the near-coincidence of the oscillator clock signal and drive enable signal. However, the result is a limit on the system power output to a percentage of the cycle set by the value of the off-time. In other words, since the cycle time is fixed, the maximum PWM ratio is set by the fixed off time.
While it would appear that the above mentioned disadvantages could be overcome by changing the oscillator frequency, such changes may result in increased switching losses (thermal dissipation) in the power switching devices driven by the PWM system, i.e., thermal dissipation increases with increases in switching frequency. Fixed clock rate PWM systems are therefore preferred in order to limit switching losses to predetermined maximum values. However, fixed clock rate circuits also introduce an unfavorable ratio of peak current to average current in ECM control systems since the clock rate can only be optimized for either low speed or high speed operation but not both.
In some applications the switching rate of a PWM system can be limited by the rise and fall time of load current. However, in an ECM system, the rise and fall time of the load current varies over a wide range as a function of motor speed and is therefore not reliable for limiting switching rates. Even if switching rates were predictable, a low inductance short-circuit would result in unacceptable high switching rates.
Another disadvantage in ECM systems is that motor circulating current is not available for direct observation when some of the power switching devices, for example, the lower rail devices in a full bridge circuit, are being switched under PWM control. This disadvantage requires reliance on either a fixed clock rate PWM circuit or a fixed off-time circuit to reinstate conduction after a preset maximum current has caused the switching devices to be turned off since the magnitude of current decay in the motor windings cannot be monitored. A fixed off-time circuit is capable of a timed off-interval consistent with an optimal current decay for high speed motor operation but allows too high a switching rate at low speed motor operation thus requiring switching with higher thermal capacity. In comparison, a fixed clock rate PWM circuit limits the switching rate of power control devices at all motor speeds. However, at high speeds the rate of current decay in the motor windings is fast. At a fixed clock rate, the current will have dropped to an unsatisfactory low level before a switch is again rendered conductive.